Objective: With dental bonding systems that require acid etching of dentin, inadequate adhesive penetration can leave exposed collagen at the dentin-adhesive interface. The exposed collagen could be degraded by bacterial proteases, compromising the integrity of the dentin-adhesive bond and, ultimately, the restoration. The purpose of this study was to develop a nondestructive staining technique to identify exposed collagenous protein at the dentin-adhesive interface. Method and materials: The following adhesives were placed, according to manufacturer's instructions, on dentin cut from 15 human third molars: Scotchbond Multi-Purpose, Scotchbond Multi-Purpose Plus, and 3M Single Bond. Light microscopic sections of native dentin-adhesive interfaces of each tooth were cut and stained with Goldner's trichrome. This reagent stained exposed protein in the sections a distinct red-orange. Results: Exposed protein was identified at the dentin-adhesive interface with each of the adhesives. Corollary scanning electron microscopic examination confirmed the presence of exposed protein, ie, protein that was removed by sodium hypochlorite, at the interface. Sites of exposed protein that were clearly identified in the light microscopic sections were obscured in the transmission electron microscopic sections. Conclusion: In vitro identification of inadequacies in the dentin-adhesive bond is the first step in determining sites that may be vulnerable to premature breakdown under clinical conditions.